At the BaselWorld fair of 2000, Rolex introduced a new Daytona – the reference 116520 – and with it, its first completely new in-house movement in over 50 years – a fully integrated, self-winding chronograph for the Daytona – the caliber 4130. (For an explanation of the Valjoux 72 based Daytona movement as we all as the Zenith-based caliber 4030 Daytona movement, please refer to part one of this series.)

The product of five years of development, it was masterfully designed for improved robustness, efficiency, accuracy, and serviceability. Rolex’s use of a high-performance “vertical clutch” chronograph coupling mechanism was a key enabler. A brief explanation…

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Consider a chronograph mechanism as a separate subsystem, which, through a “clutch,” is coupled to the movement to power the chronograph’s indicators once activated. The most widely used is a “lateral clutch,” as was found on all prior movements used in Rolex’s chronograph watches. Lateral clutch-based chronographs work well in general, and are visually very appealing; however they have a couple significant disadvantages: 1) Loss of amplitude in the balance wheel’s oscillation when the chronograph is engaged, affecting timekeeping accuracy; and 2) Backlash. When the chronograph is either started or stopped, the chronograph seconds hand typically hops, or jumps, due to the imperfect, misaligned mating of the lateral clutch’s gear with the teeth of the movement’s driving gear.

By design, the 4130’s vertically coupled approach enables absolutely precise starts and stops of the chronograph seconds hand. When started, stopped, or reset, no unwanted jitters are seen on any of the watch’s hands. The vertical clutch also allows the chronograph to run continuously for extended periods, with no impact on timekeeping accuracy. Understanding Rolex’s longstanding, noble pursuit of maximum timekeeping accuracy, their decision to use a vertical clutch makes sense.

Smart design features are found throughout. Rolex greatly simplified the chronograph’s counter system (the hour and minute counters), integrating what used to be two separate mechanisms on two sides of the movement into one unit occupying significantly less space. Rolex used this freed up space to increase the size of the mainspring barrel – providing 72 hours of power reserve versus the caliber 4030’s 54 hours. Rolex also increased the size of the balance wheel for improved timekeeping accuracy. The balance wheel is mounted to a full bridge, fixed securely on two sides for much improved shock and vibration resistance.

Compared with the 4030, the 4130 is much easier to service. A few little known “insider” facts:

- The 4130 uses 12 different screws versus more than 40 found inside the 4030.

- The 4130 has roughly 20% fewer parts than the 4030, which yes, is a good thing.

- The 4130’s vertical clutch is serviceable, unlike the vertical clutches used by other, competing and very well-known brands. It is routinely re-lubricated during servicing, but can be disassembled and repaired as well. The alternative, unserviceable vertical clutches, are untouched during servicing. They’re reinstalled as is, and become, in the words of an experienced watchmaker friend, “basically time bombs which will, of course, eventually fail.”

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- The 4130’s mainspring can be replaced without taking the movement out of the case. A 4030, on the other hand, requires a full overhaul in order to replace the mainspring.

- All of the 4130’s chronograph parts are on the movement side – only one eccentric (e.g., difficult) is required. The 4030, with three horizontal clutches spanning both sides of the movement, requires up to five different eccentric adjustments to function properly.

- The 4130’s automatic winding mechanism winds 68% more efficiently than the 4030’s, and is equipped with more reliable reversers compared with the 4030, which uses a complicated reverser that is prone to sticking.

Many respected watchmakers will confirm: The caliber 4130 set a new benchmark among high-end, self-winding chronograph movements.

Upon first glance, the 116520 appears largely unchanged from the 16520. A closer look shows some subtle but important aesthetic changes to the dial, making the caliber 4130-equipped 116520 both easy to distinguish and extremely difficult for forgers to accurately fake. The seconds indication sub-dial was moved from the 9 o’clock position to the 6 o’clock sub-dial. Now, the chronograph’s hour register was placed in-line with the 30-minute register – and both of these sub-dials were raised above the center axis of the dial by an angle of 7 degrees.

Other minor changes include the use of wider luminous hour indices, slightly longer case lugs, and mirror polishing used on the top-side of the case versus a brushed finish used on all prior stainless Daytonas.In Part 3 of the series, Paul finally reveals what we all want to know: How does the watch actually feel on the wrist?